Reminders - cannot create a subtask

...and the fix is hitting the "upgrade" button I had been ignoring and following the prompts. I mis-read the upgrade information to mean that I needed to upgrade IOS on my other devices to access the data, not the reminders data itself. It is all working properly now.

I have the same iOS and macOS installed as you, and I was able to create subtasks without issue.

My work laptop, also on the same version of MacOS, the Reminders app works fine but that is with a different icloud account.

I did find it easier, personally, to use the Information button (I in a circle) than I did with swiping.

When I tap on the information button, there is no option to "subtasks" option at all.

My advice to you would be first to try creating a brand new reminder and see if the functionality works with it. If it does, you may need to recreate existing reminders. Failing this, I would go ahead and delete the app and reinstall it.

I have tried creating a new reminder and new lists several times, on both IOS and MacOS without any improvement. I Removed the reminders app from my iphone, disconnected the other reminders apps from icloud, and then restored the app. No change. I created a new list and new reminders and still do not have the option of indenting/subtask.

Further down the line, I might look at restoring your iPhone, but I don't think we’re there yet.

This is a brand new phone just over a week old - I'm extremely reluctant to consider restoring my phone. Also, the fact that this behavior is also present on my Mac desktop seems to indicate that the problem exists with my iCloud account

I hope this helps!

Unfortunately, it hasn't yet, but I do appreciate the assistance. Is there anything else that can be done to enable the subtask functionality?

Since I've confirmed that the Reminders app works correctly on my work laptop (with my work iCloud account), I created a non-work account on that machine and connected that account with my personal iCloud account, the same one I'm using on my iPhone and desktop. The problem with being unable to create subtasks was also present on the laptop with that account (but not with the work account). Considering that application itself works fine on the same laptop, this seems to confirm the idea that this is a problem with something in iCloud, not the application.